Objective lens for endoscope

ABSTRACT

In a retro-focus type objective lens for an endoscope, its front-group concave lens system is constituted by two concave lenses, and a predetermined conditional expression is satisfied, so as to secure a sufficient length of back focus, while fully favorably correcting chromatic aberration in magnification. A four group lens configuration having five lenses comprising a front-group concave lens system composed of two concave lenses L 1  and L 2 , a rear convex lens system composed of lenses L 3  and L 5 , with an aperture stop interposed therebetween, in order to secure a long back focus at least three times the composite focal length of the whole system, satisfied is the following conditional expression: 
     
         dx≧3.0×|f.sub.F |           (1) 
    
     wherein f F  is a composite focal length of the front-group concave lens system, and dx is a distance between a rear-side principal point of the front-group concave lens system and a front-side principal point of the rear-group convex lens system.

RELATED APPLICATIONS

This application claims the priority of Japanese Patent Application No.9-86115 filed on Mar. 19, 1997, which is incorporated herein byreference.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to an objective lens disposed at a tipportion of an endoscope and, in particular, to an objective lens for anendoscope, in which a ray direction changing prism is placed on theimage surface side thereof.

2. Description of the Prior Art

In a direct-vision type endoscope employing a solid-state imagingdevice, the latter is often axially inserted and disposed at the tipportion of the endoscope. In such an endoscope, a ray direction changingprism is inserted and disposed between its objective lens andsolid-state imaging device in general.

Since the size of the ray direction changing prism is determined by theimage size, it is necessary to fully secure the distance between thelast surface of the objective lens and the imaging position betweenwhich the ray direction changing prism is inserted and disposed, i.e.,back focus.

While the image size is reduced as the solid-state imaging device ismade smaller, flare and ghost may occur unless a sufficient margin isprovided for the distance between the prism wall surface and theeffective luminous flux. Accordingly, in view of the processing accuracyof parts and their assembling accuracy, the above-mentioned distancecannot be shortened extremely, thus making it difficult to reduce theprism size in proportion to the image size. Consequently, an objectivelens having a longer back focus as compared with its focal length isnecessary.

However, in response to demands for wider angle, the objective lens foran endoscope tends to have shorter focal length even at the same imagesize, thus making it difficult to obtain sufficient back focus.

On the other hand, Japanese Patent Application No. 6-251976 JapaneseUnexamined Patent Publication No. 8-122632) proposes an objective lenssystem, whose front-group concave lens system is composed of a singleconcave lens and a single convex lens, in which chromatic aberration inmagnification is favorably corrected. This objective lens system,however, may be insufficient in terms of back focus.

SUMMARY OF THE INVENTION

In view of such circumstances, it is an object of the present inventionto provide an objective lens for an endoscope, having a sufficientlength of back focus in which a ray direction changing prism can beinserted and disposed between a solid-state imaging device axiallydisposed at a tip portion of the endoscope and the objective lens, whilechromatic aberration in magnification is sufficiently favorablycorrected.

The objective lens for an endoscope in accordance with the presentinvention is an objective lens comprising a front-group concave lenssystem, a rear-group convex lens system, and an aperture stop disposedtherebetween;

wherein the front-group concave lens system is constituted by twoconcave lenses.

Further, the objective lens in accordance with the present inventionsatisfies the following conditional expression (1) and/or the followingconditional expressions (2) and (3):

    dx≧3.0×|f.sub.F |           (1)

    ν.sub.d >50×(2.6-n.sub.d)                         (2)

    2.0<f.sub.2 /f.sub.1 <4.0                                  (3)

wherein

f_(F) is a composite focal length of the front-group concave lenssystem;

dx is a distance between a rear-side principal point of the front-groupconcave lens system and a front-side principal point of the rear-groupconvex lens system;

ν_(d) is an Abbe number of the second lens in the front-group concavelens system;

n_(d) is a refractive index of the second lens in the front-groupconcave lens system at d-line;

f₁ is a focal length of the first lens in the front-group concave lenssystem; and

f₂ is a focal length of the second lens in the front-group concave lenssystem.

Preferably, the rear-group convex lens system comprises, successivelyfrom the object side, a third convex lens having a surface with a smallradius of curvature directed onto the image surface side, and a cementedlens composed of fourth and fifth lenses one of which is constituted bya convex lens while the other is constituted by a concave lens.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a lens configuration view showing the objective lens for anendoscope in accordance with Example 1 of the present invention;

FIGS. 2A to 2E are aberration charts showing various kinds of aberrationin the objective lens for an endoscope in accordance with Example 1 ofthe present invention;

FIG. 3 is a lens configuration view showing the objective lens for anendoscope in accordance with Example 2 of the present invention;

FIGS. 4A to 4E are aberration charts showing various kinds of aberrationin the objective lens for an endoscope in accordance with Example 2 ofthe present invention;

FIG. 5 is a lens configuration view showing the objective lens for anendoscope in accordance with Example 3 of the present invention;

FIGS. 6A to 6E are aberration charts showing various kinds of aberrationin the objective lens for an endoscope in accordance with Example 3 ofthe present invention;

FIG. 7 is a lens configuration view showing the objective lens for anendoscope in accordance with Example 4 of the present invention;

FIGS. 8A to 8E are aberration chart showing various kinds of aberrationin the objective lens for an endoscope in accordance with Example 4 ofthe present invention;

FIG. 9 is a lens configuration view showing the objective lens for anendoscope in accordance with Example 5 of the present invention;

FIGS. 10A to 10E are aberration charts showing various kinds ofaberration in the objective lens for an endoscope in accordance withExample 5 of the present invention;

FIG. 11 is a lens configuration view showing an objective lens for anendoscope as a comparative example for each of the above-mentionedexamples; and

FIGS. 12A to 12E are aberration charts showing various kinds ofaberration in the objective lens for an endoscope in accordance with theabove-mentioned comparative example.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

In the following, embodiments of the present invention will be explainedwith reference to Examples 1 to 5 and a comparative example.

EXAMPLE 1

FIG. 1 is a view showing the configuration of the objective lens for anendoscope in accordance with this example.

As depicted, this objective lens has a four-group retro-focus type lensconfiguration with five lenses. Namely, it comprises, successively fromthe object side, a first concave lens L₁ having a surface with a smallerradius of curvature directed onto the image surface side, a secondconcave lens L₂ having a surface with a smaller radius of curvaturedirected onto the image surface side, a third convex lens L₃ having asurface with a smaller radius of curvature directed onto the imagesurface side, a fourth lens L₄ made of a convex lens, and a fifth lensL₅ made of a concave lens, in which the fourth lens L₄ and the fifthlens L₅ constitute a cemented lens.

An aperture stop 1 (only its position on an optical axis X beingdepicted) is disposed in the vicinity of the third lens L₃ on the objectside. The first and second lenses L₁ and L₂ constitute a front-groupconvex lens system, whereas the third, fourth, and fifth lenses L₃, L₄,and L₅ constitute a rear-group convex lens system, with the aperturestop 1 interposed therebetween. A ray direction changing prism 2 isdisposed in the vicinity of the rear-group convex lens system on theimage surface side, and a solid-state imaging device (CCD) 3 is placedin the vicinity of the ray direction changing prism 2 on the imagesurface side.

The objective lens is constituted so as to satisfy the followingconditional expressions:

    dx≧3.0×|f.sub.F |(1)

    ν.sub.d >50×(2.6-n.sub.d)                         (2)

    2.0<f.sub.2 /f.sub.1 <4.0                                  (3)

wherein

f_(F) is a composite focal length of the front-group concave lenssystem;

dx is a distance between a rear-side principal point of the front-groupconcave lens system and a front-side principal point of the rear-groupconvex lens system;

μ_(d) is an Abbe number of the second lens in the front-group concavelens system;

n_(d) is a refractive index of the second lens in the front-groupconcave lens system at d-line;

f₁ is a focal length of the first lens in the front-group concave lenssystem; and

f₂ is a focal length of the second lens in the front-group concave lenssystem.

The above-mentioned conditional expressions are set on the basis of thefollowing reasons.

Conditional Expression (1)

This conditional expression is set for obtaining sufficient back focus.

In two sheets of thin lenses, their composite focal length f isrepresented by:

    1/f=1/f.sub.a +1/f.sub.b -dx/(f.sub.a ·f.sub.b)

wherein f_(a) is a focal length of one of the lenses, f_(b) is a focallength of the other lens, and dx is a distance between the two lenses.Their back focus B_(f) is represented by:

    B.sub.f =f(1-dx/f.sub.a).

Here, assuming that the back focus Bf is three times the composite focallength f, namely,

    Bf=3f,

    3f=f(1-dx/f.sub.a),

whereby

    dx=-2f.sub.a.

In this example, assuming that

f_(F) is a composite focal length of the front-group concave lenssystem, and

f_(R) is a composite focal length of the rear-group convex lens system,

    f.sub.F =f.sub.a, f.sub.R =F.sub.b.

Since f_(F) <0, fR>0,

when

    dx≦2|f.sub.F |,

    B.sub.f ≧3f.

The foregoing relates to a case with thin lenses. In an actual opticalsystem, since the composite rear-side principal point position of therear-group convex lens system enters into the lens by substantially thecomposite focal length of the whole system, assuming that the valuetaking account of the principal point and back focus is (Bf)', when

    (Bf)'=4f

is used in place of the above-mentioned expression

    Bf=3f,

conditional expression:

    dx≧3|f.sub.F |

is obtained.

When this conditional expression is satisfied, a long back focus whichis at least three times the composite focal length of the wholeobjective lens system can be secured.

Consequently, though the ray direction changing prism 2 is inserted anddisposed between the solid-state imaging device 3 axially disposed atthe tip portion of the endoscope and the objective lens, an image of anobject can be formed on the solid-state imaging device 3.

Conditional Expression (2)

This conditional expression defines the glass material used for thesecond lens L₂ in the front-group concave lens system.

Namely, in the objective lens for an endoscope, while the glass materialused for the first lens L₁ directly in contact with the exterior islimited to the one having excellent weather resistance and chemicalresistance, thereby narrowing the degree of freedom in choice, thesecond lens L₂ has a certain degree of freedom.

In the objective lens for an endoscope, chromatic aberration inmagnification in short wavelength tends to be on the under side. Inorder to correct this tendency, the concave lens located on the objectside of the stop preferably has an Abbe number as great as possible. Onthe other hand, in order to correct coma, its refractive index ispreferably as high as possible. This conditional expression indicatestheir relationship.

Conditional expression (3)

This conditional expression defines the ratio between the focal lengthsof the first lens L₁ and the second lens L₂ in the front-group concavelens system.

Namely, it indicates that the first lens L₁ has a power stronger thanthat of the second lens L₂ by on the order of two to four times. Whenthe second lens L₂ has a weak power, correction of chromatic aberrationin magnification, which is the aimed object of the present invention,may become insufficient. When the second lens L₂ has a strong power, bycontrast, the effect of the first lens L₁ for widening angle of view isweakened, thus failing to achieve wider angle of view.

Table 1 shows radius of curvature r (mm) of each lens surface, axialsurface spacing d (mm) of each lens (center thickness of each lens andair gap between neighboring lenses), and refractive index n_(d) and Abbenumber ν_(d) at d-line of each lens in this example.

In Table 1, numbers denoting each letter successively increase from theobject side, and each value of r and d is standardized for the casewhere the focal length is 1 mm.

                  TABLE 1    ______________________________________           r     d           n.sub.d ν.sub.d    ______________________________________    1        ∞ 0.4257      1.88300                                       41.0    2        0.9977  0.2271    3        3.5535  0.4257      1.62041                                       60.3    4        1.1041  0.9650    5        Stop    0.0568    6        ∞ 1.3340      1.62041                                       60.3    7        -1.3524 0.1419    8        12.6871 1.2347      1.62041                                       60.3    9        -1.1041 0.4967      1.84666                                       23.8    10       -2.6978 0.4500    11       ∞ 4.9665      1.53996                                       59.7    12       ∞    ______________________________________

Table 2 shows the image size, object distance, angle of view, back focusB_(f), composite focal length and rear-side principal point position ofthe front-group concave lens system, front-side principal point positionand rear-side principal point position of the rear-group convex lenssystem, and focal lengths of the first lens L₁ and the second lens L₂ inthe front-group concave lens system obtained by the above-mentionedobjective lens.

                  TABLE 2    ______________________________________    Image size        1.840 mm (diameter)    Object distance   8.5149 mm    Angle of view     12° 33'    Bf = 3.567f    ______________________________________    Composite focal length of front-group concave lens system                               :-0.6948    Rear-side principal point position of front-group concave                               :-0.2610    lens system    Front-side principal point position of rear-group convex                               :1.2075    lens system    Rear-side principal point position of rear-group convex                               :-1.0441    lens system    Focal length of first lens in front-group concave                               :-1.1298    lens system    Focal length of second lens in front-group concave                               :-2.7658    lens system    ______________________________________

From these values, dx, ν_(d), and f₂ /f₁ are calculated as:

    ______________________________________            dx = 0.2610 + 1.0218 + 1.2075             = 2.4903             = 3.611 | f.sub.F  |    ______________________________________

    ν.sub.d =60.3>50×(2.6-1.62041)=49.0

    f.sub.2 /f.sub.1 =2.448

It is clear that these values satisfy conditional expressions (1), (2),and (3).

FIG. 2A to 2E are aberration charts showing various kinds of aberrationin the objective lens for an endoscope in accordance with this example.

These aberration charts show amounts of aberration at object heights of60%, 80%, and 100% when the effective Fno is 5.60.

As can be seen from these charts, this example can yield an objectivelens for an endoscope having favorable imaging performances extending tothe periphery of its visual field.

EXAMPLE 2

FIG. 3 is a view showing the configuration of the objective lens for anendoscope in accordance with this embodiment.

As depicted, this objective lens has a lens configuration substantiallythe same as that of Example 1.

Table 3 shows radius of curvature r (mm) of each lens surface, axialsurface spacing d (mm) of each lens, and refractive index n_(d) and Abbenumber ν_(d) at d-line of each lens in this example. They are indicatedin a manner similar to that of Example 1.

                  TABLE 3    ______________________________________           r      d           n.sub.d ν.sub.d    ______________________________________    1        -22.4079 0.4157      1.88300                                        41.0    2        0.9677   0.1906    3        2.1408   0.4157      1.62041                                        60.3    4        1.0150   0.9508    5        Stop     0.0554    6        ∞  1.2747      1.62041                                        60.3    7        -1.3315  0.1386    8        12.9240  1.2471      1.62041                                        60.3    9        -1.0780  0.4850      1.84666                                        23.8    10       -2.5707  1.1107    11       ∞  3.8586      1.55920                                        53.9    12       ∞    ______________________________________

Table 4 shows the image size, object distance, angle of view, back focusB_(f), composite focal length and rear-side principal point position ofthe front-group concave lens system, front-side principal point positionand rear-side principal point position of the rear-group convex lenssystem, and focal lengths of the first lens L₁ and the second lens L₂ inthe front-group concave lens system obtained by the above-mentionedobjective lens.

                  TABLE 4    ______________________________________    Image size        1.800 mm (diameter)    Object distance   8.3138 mm    Angle of view     120° 27'    Bf = 3.468f    ______________________________________    Composite focal length of front-group concave lens system                                :-0.6897    Rear-side principal point position of front-group concave                                :-0.2427    lens system    Front-side principal point position of rear-group convex                                :1.1873    lens system    Rear-side principal point position of rear-group convex                                :-1.0382    lens system    Focal length of first lens in front-group concave lens system                                :-1.0419    Focal length of second lens in front-group concave lens                                :-3.6231    system    ______________________________________

From these values, dx, ν_(d), and f₂ /f₁ are calculated as:

    ______________________________________            dx = 0.2427 + 1.0062 + 1.1873             =2.4362             =3.506 | f.sub.F  |    ______________________________________

    ν.sub.d =60.3>50×(2.6-1.62041)=49.0

    f.sub.2 /f.sub.1 =3.477

It is clear that these values satisfy conditional expressions (1), (2),and (3).

FIGS. 4A to 4E are aberration charts showing various kinds of aberrationin the objective lens for an endoscope in accordance with this example.They show the aberration in a manner similar to that of Example 1.

As can be seen from these charts, this example can yield an objectivelens for an endoscope having favorable imaging performances extending tothe periphery of its visual field.

EXAMPLE 3

FIG. 5 is a view showing the configuration of the objective lens for anendoscope in accordance with this embodiment.

As depicted, this objective lens has a lens configuration substantiallythe same as that of Example 1.

Table 5 shows radius of curvature r (mm) of each lens surface, axialsurface spacing d (mm) of each lens, and refractive index n_(d) and Abbenumber ν_(d) at d-line of each lens in this example. They are indicatedin a manner similar to that of Example 1.

                  TABLE 5    ______________________________________           r      d           n.sub.d ν.sub.d    ______________________________________    1        ∞  0.4256      1.80420                                        46.5    2        0.8513   0.1951    3        5.2650   0.4256      1.71301                                        53.9    4        1.4870   0.9735    5        Stop     0.0568    6        ∞  1.3052      1.62041                                        60.3    7        -1.3633  0.1419    8        13.2329  1.2769      1.62041                                        60.3    9        -1.1038  0.4966      1.84666                                        23.8    10       -2.6322  0.7106    11       ∞  4.6169      1.55920                                        53.9    12       ∞    ______________________________________

Table 6 shows the image size, object distance, angle of view, back focusB_(f), composite focal length and rear-side principal point position ofthe front-group concave lens system, front-side principal point positionand rear-side principal point position of the rear-group convex lenssystem, and focal lengths of the first lens L₁ and the second lens L₂ inthe front-group concave lens system obtained by the above-mentionedobjective lens.

                  TABLE 6    ______________________________________    Image size        1.800 mm (diameter)    Object distance   8.5126 mm    Angle of view     116° 15'    Bf = 3.563f    ______________________________________    Composite focal length of front-group concave lens system                                :-0.6917    Rear-side principal point position of front-group concave                                :-0.2623    lens system    Front-side principal point position of rear-group convex                                :1.2157    lens system    Rear-side principal point position of rear-group convex                                :-1.0631    lens system    Focal length of first lens in front-group concave lens system                                :-1.0585    Focal lenght of second lens in front-group concave lens                                :-3.0493    system    ______________________________________

From these values, dx, ν_(d), and f₂ /f₁ are calculated as:

    ______________________________________            dx = 0.2623 + 1.0303 + 1.2157             = 2.5083             = 3.626 | f.sub.F  |    ______________________________________

    ν.sub.d =53.9>50×(2.6-1.71301)=44.3

    f.sub.2 /f.sub.1 =2.881

It is clear that these values satisfy conditional expressions (1), (2),and (3).

FIGS. 6A to 6E are aberration charts showing various kinds of aberrationin the objective lens for an endoscope in accordance with this example.They show the aberration in a manner similar to that of Example 1.

As can be seen from these charts, this example can yield an objectivelens for an endoscope having favorable imaging performances extending tothe periphery of its visual field.

EXAMPLE 4

FIG. 7 is a view showing the configuration of the objective lens for anendoscope in accordance with this embodiment.

As depicted, this objective lens has a lens configuration substantiallythe same as that of Example 1.

Table 7 shows radius of curvature r (mm) of each lens surface, axialsurface spacing d (mm) of each lens, and refractive index n_(d) and Abbenumber ν_(d) at d-line of each lens in this example. They are indicatedin a manner similar to that of Example 1.

                  TABLE 7    ______________________________________           r     d           n.sub.d ν.sub.d    ______________________________________    1        ∞ 0.4229      1.83481                                       42.7    2        0.8458  0.3524    3        2.8194  0.4229      1.71301                                       53.9    4        1.1279  0.7616    5        Stop    0.0564    6        11.2775 1.2969      1.62041                                       60.3    7        -1.3546 0.1410    8        13.1483 1.2687      1.62041                                       60.3    9        -1.1278 0.4934      1.84666                                       23.8    10       -2.6153 0.5000    11       ∞ 4.8749      1.55920                                       53.9    12       ∞    ______________________________________

Table 8 shows the image size, object distance, angle of view, back focusB_(f), composite focal length and rear-side principal point position ofthe front-group concave lens system, front-side principal point positionand rear-side principal point position of the rear-group convex lenssystem, and focal lengths of the first lens L₁ and the second lens L₂ inthe front-group concave lens system obtained by the above-mentionedobjective lens.

                  TABLE 8    ______________________________________    Image size        1.800 mm (diameter)    Object distance   8.4581 mm    Angle of view     117° 16'    Bf = 3.517f    ______________________________________    Composite focal length of front-group concave lens system                                :-0.6254    Rear-side principal point position of front-group concave                                :-0.3173    lens system    Front-side principal point position of rear-group convex                                :1.1724    lens system    Rear-side principal point position of rear-group convex                                :-1.1728    lens system    Focal length of first lens in front-group concave lens                                :-1.0132    system    Focal length of second lens in front-group concave lens                                :-2.9431    system    ______________________________________

From these values, dx, ν_(d), and f₂ /f₁ are calculated as:

    ______________________________________            dx = 0.3173 + 1.8180 + 1.1724             = 2.3077             = 3.690 | f.sub.F |    ______________________________________

    ν.sub.d =53.9>50×(2.6-1.71301)=44.3

    f.sub.2 /f.sub.1 =2.905

It is clear that these values satisfy conditional expressions (1), (2),and (3).

FIGS. 8A to 8E are aberration charts showing various kinds of aberrationin the objective lens for an endoscope in accordance with this example.They show the aberration in a manner similar to that of Example 1.

As can be seen from these charts, this example can yield an objectivelens for an endoscope having favorable imaging performances extending tothe periphery of its visual field.

EXAMPLE 5

FIG. 9 is a view showing the configuration of the objective lens for anendoscope in accordance with this embodiment.

As depicted, this objective lens has a lens configuration substantiallythe same as that of Example 1 but differs therefrom in that the fourthlens L₄ constituting the cemented rear-group convex lens system is madeof a concave lens while the fifth lens L₅ is made of a convex lens.

Table 9 shows radius of curvature r (mm) of each lens surface, axialsurface spacing d (mm) of each lens, and refractive index n_(d) and Abbenumber ν_(d) at d-line of each lens in this example. They are indicatedin a manner similar to that of Example 1.

                  TABLE 9    ______________________________________           r     d           n.sub.d ν.sub.d    ______________________________________    1        ∞ 0.4253      1.88830                                       41.0    2        0.8220  0.1950    3        2.8353  0.4253      1.62041                                       60.3    4        0.9487  0.9727    5        Stop    0.0567    6        ∞ 1.3042      1.62041                                       60.3    7        -1.5070 0.1418    8        2.6467  1.2759      1.62041                                       60.3    9        1.1029  0.4962      1.84666                                       23.8    10       -9.8057 0.7088    11       ∞ 4.4629      1.51680                                       64.2    12       ∞    ______________________________________

Table 10 shows the image size, object distance, angle of view, backfocus B_(f), composite focal length and rear-side principal pointposition of the front-group concave lens system, front-side principalpoint position and rear-side principal point position of the rear-groupconvex lens system, and focal lengths of the first lens L₁ and thesecond lens L₂ in the front-group concave lens system obtained by theabove-mentioned objective lens.

                  TABLE 10    ______________________________________    Image size        1.800 mm (diameter)    Object distance   8.5059 mm    Angle of view     116° 9'    Bf = 3.541f    ______________________________________    Composite focal length of front-group concave lens system                                :-0.5749    Rear-side principal point position of front-group concave                                :-0.2425    lens system    Front-side principal point position of rear-group convex                                :0.8748    lens system    Rear-side principal point position of rear-group convex                                :-1.1926    lens system    Focal length of first lens in front-group concave lens                                :-0.9310    system    Focal length of second lens in front-group concave lens                                :-2.5153    system    ______________________________________

From these values, dx, ν_(d), and f₂ /f₁ are calculated as:

    ______________________________________            dx = 0.2425 + 1.0294 + 0.8748             = 2.1467             = 3.734 | f.sub.F |    ______________________________________

    ν.sub.d =60.3>50×(2.6-1.62041)=49.0

    f.sub.2 /f.sub.1 =2.702

It is clear that these values satisfy conditional expressions (1), (2),and (3).

FIGS. 10A to 10E are aberration charts showing various kinds ofaberration in the objective lens for an endoscope in accordance withthis example. They show the aberration in a manner similar to that ofExample 1.

As can be seen from these charts, this example can yield an objectivelens for an endoscope having favorable imaging performances extending tothe periphery of its visual field.

Comparative Example

FIG. 11 is a view showing, as a comparative example for theabove-mentioned Examples 1 to 5, the configuration of a four-lensobjective lens for an endoscope, in which the front-group concave lenssystem is constituted by a single concave lens.

Table 11 shows radius of curvature r (mm) of each lens surface, axialsurface spacing d (mm) of each lens, and refractive index n_(d) and Abbenumber ν_(d) at d-line of each lens in this comparative example. Theyare indicated in a manner similar to that of Example 1.

                  TABLE 11    ______________________________________    r            d           n.sub.d ν.sub.d    ______________________________________    1       -13.4697 0.4041      1.88830                                       41.0    2       0.7770   1.4816    3       Stop     0.0539    4       -10.7757 1.0512      1.71301                                       53.9    5       -1.7152  0.1347    6       11.8525  1.2123      1.71301                                       53.9    7       -1.2266  0.4714      1.84666                                       23.8    8       -2.9232  0.8113    9       ∞  4.3171      1.55920                                       53.9    10      ∞    ______________________________________

Table 12 shows the image size, object distance, angle of view, and backfocus B_(f) obtained by the above-mentioned objective lens.

                  TABLE 12    ______________________________________    Image size        1.740 mm (diameter)    Object distance   8.0818 mm    Angle of view     119° 15'    Bf = 3.465f    ______________________________________

FIGS. 12A to 12E are aberration charts showing various kinds ofaberration in the objective lens for an endoscope in accordance withthis example. They show the aberration in a manner similar to that ofExample 1.

As can also be seen from these charts, each of the above- mentionedExamples 1 to 5 greatly improves chromatic aberration in magnificationas compared with the comparative example.

As explained in the foregoing, in the objective lens for an endoscope inaccordance with the present invention, when the conditional expressionof:

    dx≧3.0×|f.sub.F |

is satisfied, a long back focus at least three times the composite focallength of the whole objective lens system can be secured.

Consequently, even in the case where a ray direction changing prism isinserted and disposed between the solid-state imaging device axiallydisposed at the tip portion of the endoscope and the objective lens, theimage of an object can be formed on the solid-state imaging device.

Also, since the front-group concave lens system is constituted by twoconcave lenses, the foregoing effects can be obtained while chromaticaberration in magnification is sufficiently favorably corrected.

Specifically, when conditional expressions of

    ν.sub.d >50×(2.6-n.sub.d)

    2.0<f.sub.2 /f.sub.1 <4.0

are satisfied, the objective lens for an endoscope in accordance withthe present invention can achieve a wider angle of view, and correctcoma and chromatic aberration in magnification while securing a longback focus.

What is claimed is:
 1. An objective lens for an endoscope, saidobjective lens comprising a front-group concave lens system, arear-group convex lens system, and an aperture stop disposedtherebetween;wherein said front-group concave lens system is constitutedby two concave lenses; and wherein said objective lens satisfies thefollowing conditional expression (1):

    dx≧3.0×|f.sub.F |           (1)

wherein f_(F) is a composite focal length of said front-group concavelens system; and dx is a distance between a rear-side principal point ofsaid front-group concave lens system and a front-side principal point ofsaid rear-group convex lens system.
 2. An objective lens for anendoscope according to claim 1, further satisfying the followingconditional expressions (2) and (3):

    νd>50×(2.6-n.sub.d)                               (2)

    2.0<f.sub.2 /f.sub.1 <4.0                                  (3)

wherein νd is an Abbe number of the second lens in said front-groupconcave lens system; nd is a refractive index of the second lens in saidfront-group concave lens system at d-line; f₁ is a focal length of thefirst lens in said front-group concave lens system; and f₂ is a focallength of the second lens in said front-group concave lens system.
 3. Anobjective lens for an endoscope according to claim 2, wherein saidrear-group convex lens system comprises, successively from the objectside, a third convex lens having a surface with a small radius ofcurvature directed onto the image surface side, and a cemented lenscomposed of fourth and fifth lenses one of which is constituted by aconvex lens while the other is constituted by a concave lens.
 4. Anobjective lens for an endoscope, and objective lens comprising afront-group concave lens system, a rear-group convex lens system, and anaperture stop disposed therebetween;wherein said front-group concavelens system is constituted by two concave lenses; and wherein saidobjective lens satisfies the following conditional expressions (4) and(5):

    νd>50×(2.6-nd)                                    (4)

    2.0<f.sub.2 /f.sub.1 <4.0                                  (5)

wherein νd is an Abbe number of the second lens in said front-groupconcave lens system; nd is a refractive index of the second lens in saidfront-group concave lens system at d-line; f₁ is a focal length of thefirst lens in said front-group concave lens system; and f₂ is a focallength of the second lens in said front-group concave lens system.
 5. Anobjective lens for an endoscope according to claim 4, wherein saidrear-group convex lens system comprises, successively from the objectside, a third convex lens having a surface with a small radius ofcurvature directed onto the image surface side, and a cemented lenscomposed of fourth and fifth lenses one of which is constituted by aconvex lens while the other is constituted by a concave lens.
 6. Anobjective lens for an endoscope, said objective lens comprising afront-group concave lens system, a rear-group convex lens system, and anaperture stop disposed therebetween;wherein said front-group concavelens system is constituted by two concave lenses; wherein said objectivelens satisfies the following conditional expression (1):

    dx≧3.0×|f.sub.F |           (1)

wherein f_(F) is a composite focal length of said front-group concavelens system; and dx is a distance between a rear-side principal point ofsaid front-group concave lens system and a front-side principal point ofsaid rear-group convex lens system; and further satisfying the followingconditional expression (2):

    νd>50×(2.6-nd)                                    (2)

whereinνd is an Abbe number of the second lens in said front-groupconcave lens system; nd is a refractive index of the second lens in saidfront-group concave lens system at d-line.
 7. An objective lens for anendoscope, said objective lens comprising a front-group concave lenssystem, a rear-group convex lens system, and an aperture stop disposedtherebetween;wherein said front-group concave lens system is constitutedby two concave lenses; wherein said objective lens satisfies thefollowing conditional expression (1):

    dx≧3.0×|f.sub.F |           (1)

wherein f_(F) is a composite focal length of said front-group concavelens system; and dx is a distance between a rear-side principal point ofsaid front-group concave lens system and a front-side principal point ofsaid rear-group convex lens system; and further satisfying the followingconditional expression (2):

    2.0<f.sub.2 /f.sub.1 <4.0                                  (2)

wherein f₁ is a focal length of the first lens in said front-groupconcave lens system; and f₂ is a focal length of the second lens in saidfront-group concave lens system.
 8. An objective lens for an endoscopeaccording to claim 6, wherein said rear-group convex lens systemcomprises, successively from the object side a third convex lens havinga surface with a small radius of curvature directed onto the imagesurface side, and a cemented lens composed of fourth and fifth lensesone of which is constituted by a convex lens while the other isconstituted of a concave lens.
 9. An objective lens for an endoscopeaccording to claim 7, wherein said rear-group convex lens systemcomprises, successively from the object side a third convex lens havinga surface with a small radius of curvature directed onto the imagesurface side, and a cemented lens composed of fourth and fifth lensesone of which is constituted by a convex lens while the other isconstituted of a concave lens.
 10. An objective lens for an endoscopeaccording to claim 1, wherein said rear-group convex lens systemcomprises, successively from the object side a third convex lens havinga surface with a small radius of curvature directed onto the imagesurface side, and a cemented lens composed of fourth and fifth lensesone of which is constituted by a convex lens while the other isconstituted of a concave lens.